The 33-year-old flanker played 66 Tests for England, his last coming against South Africa in June 2018.
Last week, RugbyPass revealed that Robshaw is set for talks in New York this week ahead of a potential move to Major League Rugby.
Robshaw has made 203 appearances for Quins, captaining the side a record 92 times, and twice won the Premiership Player of the Year award during his time at the Stoop.
He is one of only two players to appear over 200 times for the club.
Robshaw wrote on the Harlequins website: “As a boy, wearing my oversized Harlequins shirt, I could never have imagined I would be fortunate enough to play for the team I admired the most.
— Chris Robshaw (@ChrisRobshaw) February 3, 2020
“Which is why it has been a particularly difficult decision for me to leave the club after 16 extraordinary years.
“I will be hanging up my Harlequins shirt at the end of the season. And though there is so much I will miss about playing for this team, I will always be eternally grateful and proud for having had the chance to stand alongside such extraordinary players.
“All the while, being supported by the best fans in the world.”
Robshaw was named England captain for the first time by Stuart Lancaster for the 2012 Six Nations, over two-and-a-half years on from his only international cap.
He led England in their 2015 World Cup campaign on home soil – which ended in pool stage elimination – and he was replaced as skipper by Dylan Hartley in January 2016, but retained his place in the side on the back of impressive club form.
It hasn't taken the ex-Wallaby long to secure alternative employment following last week's TV setback, and other star names could follow him to America in 2021 https://t.co/p5YF4odhqV
— RugbyPass (@RugbyPass) January 28, 2020
Robshaw made no reference to retiring in his statement, hinting he intends to extend his playing career with a different club.
He continued: “I would like to take this opportunity to thank all the coaches and everyone at Harlequins who have helped me achieve my dreams, the Harlequins Academy and my incredible team-mates without whom none of this would have been possible.
“Despite all the intense training hours, and the emotional and physical demands of the job, it never felt like work.
“Putting on the Harlequins shirt will always remain the greatest privilege of my professional career.
"Your Fodens and all these other players…They kind of disappear into the background whereas this guy absolutely does not.”
— RugbyPass (@RugbyPass) February 2, 2020
“I look forward to the next chapter in my life, and will keep you posted. But wherever I end up next, I will do my utmost to make you proud.
“The Stoop & Harlequins will always be my home. Let’s enjoy the next few months.”
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